A basic diagram of the important elements of a typical induction motor is shown in FIG. 1. The motor 100 includes a rotor 103, a start winding 102, and a main winding 101. Also included are an optional capacitor 104 and a start winding switch 105.
In operation, the start winding 102 and the main winding 101 are utilized to start the motor. The start winding 102 is required in addition to main winding 101 in order to generate a rotating torque field until the rotor 103 is traveling at sufficient speed in order to generate its own field. At that point, the start winding is typically switched out of the system by some type of detection circuit which opens switch 105.
Optional capacitor 104 may be used to assist in creating a phase difference between the fields produced by start winding 102 and main winding 103. Other induction motors (not shown) include an additional capacitor which is always in the circuit.
One issue of importance is the reinstatement of the start winding 102 after the motor is running. More specifically, if the load on the motor increases, the rotation of the rotor 103 will slow. As the rotor 103 slows down, the torque field it generates decreases, eventually reaching the point where it is insufficient to keep the rotor 103 rotating. Accordingly, it is required that the start winding be reinstated.
If the start winding is reinstated after the motor has slowed too much, motor stall will result and the motor may not be able to overcome the overload condition which caused the stall. If the start winding is reinstated when the rotor 103 is moving too fast, then the motor may be damaged. Accordingly, it is important that the start winding be reinstated at the appropriate rotor speed during increased load. Specifically, as load increases and rotor speed decreases, the start winding should be reinstated at approximately 30 percent of full speed, with a tolerance of approximately 10 percent.
U.S. Pat. No. 4,804,901 describes a circuit for disconnecting the start winding and for reconnecting the start winding when the motor experiences an increase in load and thus, a motor stall. The '901 patent is incorporated herein by reference.
Typically, the main winding experiences maximum current when the rotor is at a stand still. The main winding current at the point where the rotor 103 stops moving is called the lock rotor current. As the speed of rotor 103 increases, current through main winding 101 decreases from the lock rotor value to a lower value. It is desirable to disconnect the start winding 102 when the rotor 103 is at approximately 75 percent full speed. This corresponds, as described in the '901 patent, to a current in main winding 101 of approximately 80 percent of the lock rotor current.
Thus, when the rotor 103 begins turning, it generates an initial current. As the speed of revolution increases, the current starts to decay. When the main winding current reaches a point at 80% of its initial value, the rotor is running at approximately 75% speed of revolution. It is at this point that the start winding is to be initially disconnected from the circuit as the field generated by the rotor 103 is sufficient to keep motor 100 running.
One way of reinstating the start winding 102 when the motor becomes loaded is to store a value equal to a particular percentage of the initial main winding motor current, say 80 percent. As the rotor 103 rotates, if a load is placed upon such rotor such that the rotor slows down, the main winding current will increase above 80 percent of its initial value. At such point, the start winding may be reinstated.
The problem with such a system is that the 80 percent value is acceptable when the motor is initially started in order to disconnect the start winding. Specifically, when the main winding current falls below a value 80 percent of its initial value, the start winding is disconnected. However, during operation, variations in the motor operating environment such as temperature and power supply and variations in the motor characteristics such as the main winding impedance, can cause the initial 80 percent value to be too high for proper reinstatement and possibly prevent proper reinstatement of the motor start winding. The 80 percent value can also be too low, therefore causing the start winding to be reinstated too soon, prematurely. Additionally, the 80 percent value may decay, in analog implementations, rendering the entire arrangement inaccurate.
Storing the main winding current reference value digitally so that it does not decay would represent one solution, however, this increases the cost and is thus less desirable. The varying motor envelope conditions will not be adapted to with this method as stated previously.
In view of the above, there exists a need in the art for an improved technique of disconnecting a start winding as motor speed increases and then reinstating the start winding when a motor load decreases rotor speed.